Charles Brown is a professor in the Department of Biological Sciences at the University of Tulsa. / Valerie O'Brien

by Cathy Payne, USA TODAY

by Cathy Payne, USA TODAY

Fewer cliff swallows are being killed by moving vehicles because of evolution, suggests a study published online today in the journal Current Biology.

"These birds have been exposed to vehicles and roads for 30-plus years," says Charles Brown, the study's lead author. "During that time, they have evolved to avoid being killed by traffic. Evolution can happen very rapidly, and some animals can adapt to urban environments very rapidly."

The decrease in road deaths is likely because these birds have shorter wingspans, making them more agile fliers, or they are learning to avoid vehicles, Brown says.

In the western USA, cliff swallows, which live in colonies, have nests around highway bridges, overpasses and road culverts. They sit on roads to pick up gravel for their gizzards to help digest food and to sunbathe.

The road kill index decreased from about 20 cliff swallows a year at the start of the study in 1983 in Nebraska to an estimated two birds a year by the study's end in 2012. This is only an estimate of those killed because more died but were not found.

The drop can't be explained by declines in the bird population or in traffic, the study suggests. The birds that continue to die on the roads have longer wingspans. Wing lengths vary between 104 mm (4.1 inches) and 115 mm (4.5 inches), Brown says.

"Longer-winged swallows sitting on a road probably can't take off as quickly, or gain altitude as quickly as shorter-winged birds, and thus the former are more likely to collide with an oncoming vehicle," says Brown, a professor of biological sciences at the University of Tulsa.

Geoff LeBaron, an ornithologist with the National Audubon Society, says shorter wingspan would allow cliff swallows to turn more quickly to avoid being hit by vehicles. "If the longer-winged birds are the ones being clobbered, then the shorter-winged birds are the ones passing on genes to the next generation," adds LeBaron, the society's Christmas Bird Count director, who was not involved in the study.

The study also found that the average wing length is shorter. In the 1980s, the average was 110 mm (4.3 inches); it dropped to 103 mm or 104 mm (4.1 inches) in the late 2000s, Brown says.

Brown says cliff swallows do learn from watching others so some may figure out how to avoid oncoming traffic. "Birds that have the ability to learn are more likely to survive and produce more babies," he says. "Over time, the population will have smarter birds."